Electrodeposition of chromium



ELECTRODEPOSITION or CHROMIUM Henry Brown, Huntington Woods, Mich., assignor, by

mesne assignments, to The Udylite Research Corporation, Detroit, Mich., a corporation of Michigan No Drawing. Application January 29, 1953, Serial No. 334,081

18 Claims. Cl. 204-51 This invention relates to improvements in the electrodeposition of chromium from aqueous acidic hcxavalent chromium baths and more particularly concerns new exceedingly stable additives for such baths which are capable of substantially reducing the formation of spray and mist during electrolysis.

The electrodeposition of chromium from aqueous acidic hexavalent chromium solutions takes place, as iswell known, with the evolution of relatively large quantities of hydrogen at the cathode and of oxygen and ozone at the insoluble anodes. The undesirable spray and mist; of chromic acid results from the violent bursting of the multitudinous gas bubbles of high surface energy which are released by the electrolysis. The spray and mist resulting from this bubble bursting is of considerable volume because of the high current densities used in the plating, the low efficiency of chromium deposition and the use of insoluble anodes. Due to the corrosiveness and high toxicity to the workers of this spray and mist, and its well known deleterious contaminating effect on other plating baths such as nickel, copper, cadmium and zinc, it is necessary in large scale production to employ powerful ventilation to continuously remove the same as it is formed. For example, to carry away the mist formed in a 2,000 gallon tank installation requires an exhaust of as much as 10,000 cubic feet per minute to enable safe continuous production. The use of powerful ventilation is expensive not only from the standpoint of the initial installation cost and upkeep maintenance, but also because of the abstraction of the heated air in winter. Moreover, not all of the spray and mist is removed from the area of the plating tank even when powerful ventilation is employed and this is especially true on wide plating tanks. In the usual installation, a certain amount of chromic acid mist does contaminate the air, especially when stray air currents pass over the tank (luring electrodeposition. toxicity, the maximum safe concentration of chromium trioxide is now considered to be 0.1 milligram per cubic meter of inhaled air. In addition to the disadvantages connected with adequate ventilation, there is the further disadvantage of the loss of chromic acid in the exhausted air which may amount to about 30% of the chromic acid which is used in the elcctrodeposition. The amount of chromic acid thus exhausted, together with that which is carried out on the surface of the articles themselves,

actually represents more chromic acid than that which is used up in the formation of the chromium coating.

It is, therefore, the principal object of the invention to provide a stable additive for an acidic hexavalent chromium bath which greatly minimizes the formation of spray and mist during electrolysis of such baths with insoluble or highly polarized anodes.

Another important object of this invention is to provide improved acidic, hexavalent chromium baths and a method for chromium plating which substantially eliminates the necessity for the use of expensive ventilating procedures currently employed.

From the standpoint of nited States Patent Patented June 12, 1956 An additional object is to improve the. operating efficiency of hexavalent chromium baths. Other objects will become apparent hereinafter.

The problems arising as the result of the formation of chromic acid spray and mist in commercial chromium plating, were recognized almost from the very inception of chromium plating and various attempts have been made to prevent or to greatly minimize, the formation of this spray and mist. As examples of attempted so-v lutions, it has been suggested to, form blankets on the surface of the bath by the use of various oils, floating objects such, as plastic pieces, and various wetting agents especially non-ionic wetting agents. None of these attempts have been found to be eminently satisfactory, and allhave left much to be desired. For example, non-ionic wetting agents are rapidly oxidized at the anode, and floating plastic pieces are too readily displaced when articles are placed in and taken out of the bath.

It has now been found that the highly surface-active p u s h invention, as exemp ified by t e o poundsv of Table I, which carry a fluorinated carbon chain and a sul n c group are mpl tely stable to the highest oxidation potentials existing during electrolysis with insoluble anodes in acidic solutions of hexavalent chromium. Furthermore, these compounds resist the most acid and high temperature conditions existent in chromium plating baths. The compounds of this invention are capable of preventing or greatly minimizing the fine spray and mist which results from the evolution of the gases at the electrodes. Heretofore, no compound which is soluble in the bath has beenfound that will prevent or effectively minimize the formation of spray and mist and yet be stable to the extremely powerful oxidizing conditions existing at the insoluble anodes during the eleetrodeposition of chromium from the acidic hexavalent chromium baths.

The fluorocarbon sulfonic compounds of this invention are usable in aqueous acidic hexavalent chromium plating baths of conventional composition and have been found to lower the surface tension of such solutions to amazingly lQW values and cause the formation of small, as opposed to large bubbles of electrolytically evolved gases. (Toncurrently, the compounds of this invention apparently impart sufficient surface viscosity to the bubbles which, in combination with their low surface energy, prevents. heir violent. collapse at the surface of the bath. The fluorocarbon sulfonic compounds of this invention can b har ct zed by h r u a h re naf e de nated Formula I where Rn represents a saturated fluorocarbon chain including straight, branched and cyclic fluorocarbon chains of 4 to 18 carbon atoms, and X is a cation. The cation X may represent hydrogen or may be a metallic ion from the groups including the alkali metals, alkaline earth metals, rare earth metals and heavy metals, such as NI-Li, Mg, Zn, Ca, Cr, Al, Ni, Cu, Ce, etc. These compounds can be considered to be a special type of anionic surfaceactive g and in th accomplis m t of the o j of this i n on th an onic radica c s tutes h dominant surface-active portion. In comparison to the anionic radical, the cation is relatively unimportant even though additional quantities of certain cations have been observed to increase the stability of the foam, such as zinc and copper.

Typical representatives of the compounds covered by Formula I which are suitable as additives to hexavalent chromium baths and the optimum concentrations for use in such baths are given in Table I.

Table I Optimum Flnorocarbon Sulfonic Compound Structural Formula Cone grams/liter Perfluoro butyl sulfonic acid onw'rmoris 0311 6-25 Perfiuoro isoamyl sulfonic acid. 0 F 8 0; 1-6 Perfiuoro n-hexyl sulfonic acid.-. OF (CFz)4OFzSO H 0. 2-4 Peri-luoro n-heptyl sulfonic acid CF (OF)5OF;SO5H 0. 2-4 Perfiuoro n-octyl sulfonic acid- CF3(OFZ)QCFZSO3H 0. 05-4 Perfluoro n-dccyl sulfonic acid. CF3(CF2)3CF1SO3H 01-2 Perfluoro n-lauryl sulfonic acid cF (CF2) uGF2SO3H .01-2 Perfluoro cyclohexane sulfonic acid CuFuSO3H 0. 2-12 Perfluoro (4-methyl cyclohexane) sulfonic acid C7F1zSOaH 0. 2-12 Perfiuoro (Z-methyl cyclohexane) sulfonic acid C1F13SO;H 0. 2-12 Perfluoro cyclohexane methyl alphasulfonic acid C1FiaS 03H 0. 2-12 Perfluoro dimethyl cyclohexane sulfonic acids CsFmS 03H 0. 1-6 Perfluoro ethyl cyclohexane sulfonic acid 015F158 03H 0. 1-6 Perfluoro isopropyl cyclohexane sulfonic acid GOF17SO3H 0. 1-6 Perfiuoro diethyl cyclohexane sulionic acids. GmFmSOzH 01-2 Perfluoro d1- acids C zFnSOaH 01-2 Perfluoro di-(cyclohexane) di-suliouic 0' S C11Fm(SOaH)s 01-4 Perfluoro naphthalene di-sulfonic aci s OmF1e(SO3H): .01-4 Perfluoro naphthalane suifonic acids. 010F118 03H 01-2 H CF;(C F=)+-SOBH 0. 01-4 (n=2-i1) H i i CF:(OF9)t I NC-(FSO:H 0. 2-2

H H i t OF3(oFg)5-(l]NC-(]:S03H 0. 2-2

H i CFa(CF:)a -%N-C-('3-S0:H 0. 1-1. 5

O (13H; CF:(CFz)z- N-C:Hr-SO:H 4-6 a i H H GF;(CF;)4GN-C- l-SOaH I l H The fluorocarbon sulfonic compounds of this invention can be further characterized by having a chain of fluorinated carbon atoms joined to the sulfur atom in the molecule and containing not less than 4 and not more than 18 carbon atoms per sulfonic group. Compounds containing more than about 18 carbon atoms per sulfonic groups are insufliciently soluble in the bath to produce the desired result unless the temperature of the bath is increased to a point above that generally used commercially. It will be understood that the proportions set forth in Table I represent only optimum concentrations and that concentrations up to saturation may be used with good results.

The additives of this invention illustrated in Table '1 may be made in accordance with the method set forth -in U. S. Patent 2,519,983.

To illustrate that process in greater detail, a suitable procedure for making perfluoromethyl cyclohexane sulfonic acid will be set forth. Paratoluene sulfonyl chloride is utilized as the starting material and is inserted in an electrolytic cell of the type described in U. S. Patent 2,519,983 containing liquid hydrogen fluoride, and the mixture is electrolyzed for a period of hours to produce perfluoro 4-methyl eyclohexane sulfonyl fluoride. By hydrolyzing the c'yclohexane the hexavalent chromium baths.

other lead alloy anodes. trolysis, 1 gram per liter of these non-fluorinated sulfonic acids is oxidizedaway leaving inorganic sulfate as a decomposition product in the bath. The sulfate thus formed sulfonyl fluoride, the corresponding perfiuoro sulfonic acid or a salt of the latter is produced.

The new soluble fluorocarbon sulfonic compounds of Table I show an unexpected degree of surface activity in For example, the incorporation of a small quantity of about .2 gram to 1 gram per liter of the relatively simple perfluoro 4-mcthyl cyclohexane sulfonic acid greatly minimizes and substantially prevents the formation of fine spray and mist, permanently in the absence of drag-out, from a chromic acid electroplating bath operating at room temperature. Although an ordinary non-fluorinated aliphatic sulfonic acid having a carbon chain containing at least 8 carbon atoms or an aromatic sulfonic acid having at-least an 8 carbon alkyl chain attached to the benzene ring is fairly stable when merely dissolved in acidic hexavalent chromium plating baths, such materials are rapidly and completely oxidized when the plating solution is eleetrolyzed using insoluble anodes, such as lead, lead-tin, lead-antimony or In less than four hours of elecalters the usual ratio of chromic acid anhydride (CrOs) amass;

to sulfate and thus interferes with the plating range of the bath unless the excess sulfate is removed by precipitation. It will be apparent that if other catalyst" radicals are being employed in the bath with or instead of sulfate, such for example as fluoride or fluosilicate, that the formation of excess sulfate ion will correspondingly atfect the chromic acid anhydride ratio to the catalyst radical with a comparable decrease in the plating range of the bath. Because of the increased difiiculties of control of the bath caused by the breakdown in the bath of such non-fluorinated sulfonic acids, their use is undesirable. In contrast, the fluoro-carbon sulfonic compounds of the type covered by Formula I when dissolved in the bath, completely resist oxidation during electrolysis even when extremely high potentials, temperatures or concentrations of chromic acid are employed.

In a large proportion of commercial chromium plating, the chromium is applied over an underlayer of nickel. Where nickel is the underlayer, the fluorocarbon sulfonic compounds of this invention tend to render the underlying nickel plate less sensitive to passivation by contact with the chromic acid solution before current is applied. In forming a chromium layer over nickel, it has been found to be desirable in some instances to preliminarily cathodically gas (at low voltage) the nickel plated articles as it is immersed in the hexavalent chromium plating bath and before the higher electroplating voltage is applied. Such mild cathodic gassing is accomplished by using voltages below the plating voltage for a short period of time. One suitable condition for such gassing is a potential of 2 to 3 volts maintained for about 5-30 seconds.

The incorporation of the fluorocarbon compounds of this invention in conventional hexavalent chromium plating baths has also been found to enable the plating of thicker bright chromium deposits over a bright surface before dulling of the chromium plate sets in. The thicker chromium plate increases the corrosion protection by decreasing the porosity of chromium plate. For example, steel that is plated with 1-1.3 mils of bright nickel and overlaid with .05 to .08 mil of chromium, provides greatly superior corrosion protection in industrial atmos pheres than is afforded when the overlayer of chromium has a thickness of only about .02 mil. It has also been found that trivalent chromium which is formed by reduction of hexavalent chromium at the cathode, tends to maintain itself at a higher level (for example, 1-3 grams per liter) in the chromium plating bath than when no fluorocarbon sulfonic compound is present, and furthermore there is a definite increase in cathode efficiency.

In contrast to the fluorocarbon sulfonic compounds of this invention, if a fluorocarbon carboxylic acid, for example, the compounds, perfluorocaproic acid and perfluorocaprylic acid CF3(CFz)aCOOI-I or their salts is added to chromic acid baths used for the electrodeposition of chromium, for example, baths containing the ratio of 100:1 of CrOs to S04, there results a darkish or much more dull chromium deposit, that is, there is a marked decrease in brightness of the deposit which can be overcome only by the use of an unusual ratio of CrOa to S04, for example, 50:1 instead of the usual 100:1.

The compounds of Table I are exhausted from the bath only by drag-out in the film which remains on the articles being plated as the articles are removed from the bath. While the amount of drag-out can be minimized by the use of drag-out tanks or ion exchangers, the fluorocarbon sulfonic compounds of this invention containing 3-8 carbon atoms produce a suprisingly low amount of drag-out. Baths have been run for months with only negligible loss from drag-out of the compounds of Table I such as perfluoro 4-methyl cyclohexane sulfonic acid.

The shorter chain fluorocarbon sulfonic compounds of Table I, that is those containing 8 or less carbon atoms when used in concentrations of, for example, 0.5 gram/ liter up to saturation, produce rapidly collapsing foams during electrolysis while concurrently lowering the surface tension of the baths to values as low as 20-30 dynes/cm. The shorter chain compounds produce on the surface of the bath, either no visible foam blanket or a comparatively thin one during continuous electrodeposition, the quantity of foam blanket and its thickness being dependent upon the particular compound employed, its concentration, the current concentration used, the ratio of surface to volume of the bath, and the temperature of the bath. In general, as the length of the fluorocarbon chain of the compounds used and the amount thereof is increased, the tendency for the formation of a thicker foam blanket increases. As the temperature of the bath is increased, the bubbles which are formed tend to collapse more rapidly and the thickness of the foam blanket is thus reduced, These short chain compounds containing up to about 8 fluorocarbons in the chain produce a thinner foam blanket than is formed from the use of the fluorocarbon sulfonic compounds having a larger number of carbon atoms in the ring or chain. Because of the evolution of the relatively large quantities of hydrogen and oxygen, it is desirable to employ a quickly collapsing foam and to maintain on the surface only a thin blanket of such foam. A thin blanket of foam is much safer because excessively thick blankets being filled with hydrogen and oxygen are subject to explosion. Because of their tendency to quickly collapse and form thin foam blankets, the perfluoro n-hexyl sultonic compounds, perfluoro 2- or 4-metl1yl cyclohexane sulfonic compounds, perfluoro 2- or 4-ethyl cyclohexane sulfonic compounds or perfluoro dimethyl cyclohexane sulfonic compounds are preferred and are especially preferred for hexavalent chromium baths which are operated at room temperature.

At somewhat higher operating temperatures of about F. to F. the compounds containing 8 or less carbon atoms in the chain are somewhat less effective in preventing the formation of spray and mist during electrolysis than at room temperatures. It is thought that these shorter chain compounds cause a smaller decrease in the surface tension, and also the surface viscosity and orientation of the molecules in the surface film of the bubbles is decreased at the higher bath temperatures which allows more rapid collapse of the bubbles without causing a build-up of even a very thin blanket of foam. With baths operating at temperatures above room temperature, the longer chain fluorocarbon sulfonic compounds of this invention are preferred. As above explained, as the number of carbon atoms in the compound is increased, the foam becomes more stable evidently due to more pronounced orientation of the longer chains and a thicker foam blanket results on the surface of the bath. Where the foam blanket becomes too thick, it has been found that the foam can be collapsed almost instantly by passing vapors of ammonia over the bath. In this regard, the foams which are created by the compounds of this invention are unique inasmuch as foams resulting from the use of ordinary non-fluorinated wetting agents in hexavalent chromium baths are not collapsed by ammonia vapor.

The examples given below set forth formulations of operative chromium plating baths useful for decorative or engineering purposes. It will be understood that other compounds covered by Formula I and exemplified by the compounds of TableI may be used in these typical formulations in the place of the particular examples given. Additionally, it is to be understood that mixtures of the compounds of this invention may be employed as well as the single compounds.

"7 Example I 150-250 grams/liter chromic acid (CrOa) 1.5-3 grams/ liter S04 ion 0.5-4 grams/liter perfluoro 4-methyl cyclohexane sulfonic acid (potassium salt) Temperature 20 30 C.

Cathode current density100-300 amps/sq. ft. (approximately 10-30 amps/sq. drn.)

400 grams/ liter chromic acid (CrOs) 3-4 grams/liter S04 ion 0.3-3 grams/liter perfluoro 2-, or 4-ethyl cyclohexane sulfonic acid Temperature, 20 C.-40 C.

Cathode current density-100-500 amps/sq. ft.

In forming a chromium coating on underlayers of nickel or copper, it is desirable to maintain the ratio of CrOa to S04 between about 75:1 and 150: 1. Where the underlayer is white brass (20% Cu-80% Zn) it has been found that the ratio of CrO to S; may be increased to as high as 200: 1.

Example III 400 grams/liter CrOs 2-3 grams/ liter S04 ion 1.5 grams/liter I-IzSiFs 0.1-1 gram/liter perfiuoro (mixed isomers of) isopropylcyclohexane sulfonic acids Temperature20 C.-60 C.

Cathode current density 100-400 amps/sq. ft.

Example IV 400 grams/ liter CrOs 2-3 grams/ liter S04 ion 1-2 grams/liter CaFz .01-.3 gram/liter perfluoro n-decyl sulfonic acid Temperature, 20 C.-60 C.

Cathode current density100-400 amps/sq. ft.

Example V 100 grams/liter CrOs 120-220 grams/liter NazCrzOw or KzCrzOw S04 ion at 2-4 grams/liter or catalyst equivalent in F,

SiFs ions or mixtures 0.2-4 grams/ liter perfluoro 2,4 dimethyl cyclohexane sulfonic acid Temperature, 20 C.-40 C.

Cathode current density 150-300 amps/sq. ft.

Example VI 300-400 grams/ liter CrOa 3 grams/ liter S04 anion 0.5-2 grams/ liter perfluoro 4-methyl cyclohexane sulfonic acid 5-15 grams/ liter zinc dichromate, zinc oxide, or zinc carbonate Temperature 20 C.-50 C.

Cathode current density-150-300 amps/ sq. ft.

Example VII 300 grams/liter CrOs 2 grams/ liter S04 0.2-2 grams/liter perfluoro 4-ethyl cyclohexane sulfonic acid 5-15 grams per liter of copper dichromate, or carbonate Temperatures 20 C.-50 C.

Cathode current density-l50-300 amps/sq. ft.

In the solutions of this invention, zinc ions, copper ions and mixtures of zinc and copper ions may be employed in concentrations of about 5 to 20 grams per liter, and when such concentrations are employed the stability of the foam is increased.

What is claimed is:

1. In a process of electrolysis of aqueous acidic hexavalent chromium solutions, the improvement which consists in adding to the solution a saturated fluorocarbon sulfonic compound in sufiicient amount to substantially decrease formation of spray and mist, said compound having 4-18 carbon atoms to each sulfonic group.

2. In a process of electrodepositing chromium from aqueous acidic hexavalent chromium solutions, the improvement which consists in adding to the solution a compound having the formula RFSOSX Where RF represents a saturated fluorocarbon chain of 4-18 carbon atoms, and X is a cation, said compound being added in suflicient amount to substantially decrease formation of spray and mist.

3. In a process of electrodepositing chromium from aqueous acidic hexavalent chromium solutions, the improvement which consists in adding to the solution a material having the formula RFSOSX where RF represents a saturated fluorocarbon chain of 4-18 carbon atoms, X is a cation, and about 5 to 20 grams per liter of a metallic ion selected from the group consisting of zinc, copper and mixtures thereof, said material being added in sufficient amount to substantially decrease formation of spray and mist.

4. A method of electrodepositing chromium wherein the formation of spray and mist is substantially decreased, which comprises electrolyzing an aqueous acidic hexavalent chromium solution containing at least about .01 gram/liter of a saturated flouorcarbon sulfonic compound, said compound having 4-18 carbon atoms to each sulfonic group.

5. A method of electrodepositing chromium wherein the formation of spray and mist is substantially decreased which comprises electrolyzing an aqueous acidic hexavalent chromium solution containing at least about .01 gram/liter of a compound having the formula RFS03X where RF represents a saturated fluorocarbon chain of 4-18 carbon atoms, and X is a cation.

6. A method of electrodepositing chromium over nickel which comprises the steps of incorporating in an aqueous acidic hexavalent chromium bath at least about .01 gram/liter of a saturated perfluorocarbon sulfonic compound having 4-18 carbon atoms to each sulfonic group, immersing a nickel coated article in the bath, cathodically gassing said article by applying a voltage to the bath less than that normally applied for plating, and thereafter electroplating chromium from said bath.

7. A method of electrodepositing chromium over nickel which comprises the steps of adding to an aqueous acidic hexavalent chromium solution at least about .01 gram/liter of a compound having the formula RFS03X Where RF represents a saturated fluorocarbon chain of 4-18 carbon atoms, and X is a cation, immersing a nickel coated article in the bath, cathodically gassing said article by applying a voltage to the bath less than that normally applied for plating, and thereafter electroplating chromium from said bath.

8. A method of electrodepositing chromium wherein the formation of spray and mist is substantially decreased comprising the steps of electrolyzing an aqueous acidic hexavalent chromium solution containing at least about .01 gram/ liter of a saturated perfluorocarbon sulfonic compound having 4 to 18 carbon atoms to each sulfonic group, and regulating the thickness of the foam blanket which is formed by passing ammonia vapors over the said foam blanket.

9. A bath for the electrodeposition of chromium comprising an aqueous acidic hexavalent chromium solution containing a saturated perfluorocarbon sulfonic compound in sufl'icient amount to substantially decrease formation of spray and mist, said compound having 418 carbon atoms to each sulfonic group.

10. A bath for the electrodeposition of chromium com prising an aqueous acidic hexavalent chromium solution containing a compound having the formula RFSOSX where Rn represents a saturated fluorocarbon chain of 4-18 carbon atoms, and X is a cation, said compound being added in suflicient amount to substantially decrease formation of spray and mist.

11.. A bath for the electrodeposition of chromium comprising an aqueous acidic hexavalent chromium solution containing at least about 0.01 gram/liter of a compound having the formula RFSO3X Where RF represents a saturated fluorocarbon chain of 4-18 carbon atoms, and X is a cation. v

12. A bath for the electrodeposition of chromium comprising an aqueous acidic hexavalent chromium solution containing at least about 0.1 gram per liter of perfluoro cthyl cyclohexane sulfonic acid.

13. A bath for the clectrodeposition of chromium comprising an aqueous acidic hexavalent chromium solution containing at least about .2 gram/ liter of perfluoromethyl. cyclohexane sulfonic acid.

14. A bath for the electrodeposition of chromium comprising an aqueous acidic hexavalent chromium solution containing at least about 0.05 gram/liter of perfluoro noctyl sulfonic acid.

15. A bath for the electrodeposition of chromium comprising an aqueous acidic hexavalent chromium solution containing at least about .01 gram/liter of perfluoro ndccyl sulfonic acid.

16. A bath for the electrodeposition of chromium comprising an aqueous acidic hexavalent chromium solution containing at least about .2 gram/liter of perfluoro 2- methyl cyclohexane sulfonic acid.

17. A bath for the electrodeposition of chromium comprising an aqueous acidic hexavalent chromium solution containing at least about .1 gram/liter of a perfluoro dimethyl cyclohexane sulfonic acid.

18. A bath for the electrodeposition of chromium which comprises an aqueous solution of chromic acid containing a catalyst selected from the group consisting of the sulfate ion, the fluoride ion and the fluorosilicate ion and at least about .01 gram/ liter of a compound having the formula RFSO3X where RF represents a saturated fluorocarbon chain of 418 carbon atoms, and X is a cation, and about 5 to 20 grams per liter of a metallic ion selected from the group consisting of zinc, copper and mixtures thereof.

Flett Apr. 2, 1940 Simons Aug. 22, 1950 

1. IN A PROCESS OF ELECTROLYSIS OF AQUEOUS ACIDIC HEXAVALENT CHROMIUM SOLUTIONS, THE IMPROVEMENT WHICH CONSISTS IN ADDING TO THE SOLUTION A SATURATED FLUOROCARBON SULFONIC COMPOUND IN SUFFICIENT AMOUNT TO SUBSTANTIALLY DECREASE FORMATION OF SPRAY AND MIST, SAID COMPOUND HAVING 4-18 CARBON ATOMS TO EACH SULFONIC GROUP. 